Microdurometer with automatic calculation

ABSTRACT

A microdurometer for measuring the hardness of a sample by the production of an impression therein by means of a pyramidal diamond with four faces which is applied to the sample under a predetermined force P, the hardness being proportional to P/d2 where d is the length of the diagonal of the impression, the microdurometer including means for applying the diamond to the sample, means for linearly displacing a sighting grid between the ends of an impression made in a sample, a first voltage generator coupled to the grid displacing means producing a voltage inversely proportional to the square of the displacement of the grid, a second voltage generator for producing a voltage proportional to the force of application of the diamond and connected to the first voltage generator to produce an analogue product of the two voltages proportional to the hardness of a sample and a voltmeter calibrated in units of hardness to indicate the product.

United States Patent [191 Llop MICRODUROMETER WITH AUTOMATIC CALCULATION[75] Inventor: Helenio Llop, Montreuil, France [73] Assignee:Creusot-Loire, Paris, France [22] Filed: Mar. 3, 1972 [21] Appl. No;231,490

[30] Foreign Application Priority Data Primary Examiner-Richard C.Queisser Assistant ExaminerArthur E. Korkosz Attorney-William B. Kerkam,Jr.

[ June 19, 1973 [57] ABSTRACT A microdurometer for measuring thehardness of a sample by the production of an impression therein by meansof a pyramidal diamond with four faces which is applied to the sampleunder a predetermined force P, the hardness being proportional to P/a'where d is the length of the diagonal of the impression, themicrodurometer including means for applying the diamond to the sample,means for linearly displacing a sighting grid between the ends of animpression made in a sample, a first voltage generator coupled to thegrid displacing means producing a voltage inversely proportional to thesquare of the displacement of the grid, a second voltage generator forproducing a voltage proportional to the force of application of thediamond and connected to the first voltage generator to produce ananalogue product of the two voltages proportional to the hardness of asample and a voltmeter calibrated in units of hardness to indicate theproduct.

11 Claims, 13 Drawing Figures PATENIED JUN I SW5 3, (39 630 SHEET 1 BF5.

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SHEET 6 [if 6 34) 3L L 205 i-" MICRODUROMETER WITH AUTOMATIC CALCULATIONThe present invention relates to improvements in microdurometers.

Measurement of the hardness of a sample is at present performed onapparatus called a microdurometer, by producing an impression in thesample under test by means of a four-faced pyramidal diamond applied tothe sample under a predetermined load P. By means of a micrometricmeasuring device the length of the diagonal of the impression is thenmeasured. Depending on the shape of the diamond used to make theimpression, the hardness H is expressed either on the Vickers scale bythe formula H 1854. 4P/d or on the Knoop scale by the formula H 14229P/d, in these two formulae the load P is expressed in grammes and thelengthd of the diagonal of the impression is expressed in microns.

Usual hardnesses allow of separate measurement of the load ofapplication of the diamond to the sample,

P, and the length d of the diagonal of the impression.

The hardness is then determined by the use of tables, generally suppliedwith the apparatus and giving the value of the hardness as a function ofthe variables P and d.

In apparatus of this type the cycle of production of the impression onthe sample is already usually made automatic, which eliminates the humanfactor in the mechanical performance of the impression. On the otherhand the determination of the true applied load and above all themeasurement of the length of the diagonal are measurements which leaveroom for an interpretation of the reading which can lead to slightlydifferent results of measurement, depending on the operator.

In accordance with the present invention there is provided amicrodurometer for measuring the hardness of a sample by the productionof an impression therein by means of a pyramidal impress diamondwvithfour faces applied to the sample under a force P, in which themeasurement of the hardness H is determined by a formula of the form HKP/d, d being the length of the diagonal of the impression and K aconstant coefficient, the microdurometer comprising a slide carrying animpress diamond, operating means for the slide for applying the diamondto a sample with an adjustable force, optical means for observing theimpression, means for linearly displacing a sighting grid, a firstvoltage generator coupled to the displacing means for providing anoutput signal inversely proportional to the square of the lineardisplacement of the grid and provided with means for reducing the outputsignal to zero, a second voltage generator for providing an outputsignal proportional to the force of application of the diamond, the twogenerators being connected to provide an analogue product of theiroutput signals, and a voltmeter for indicating the product of the twosignals.

According to one embodiment of the invention, applicable to amicrodurometer in which the displacement of the grid is controlled by adisc of which the angular rotation is proportional to the lineardisplacement of the grid, the first generator includes a a=l/x"'- lawpotentiometer, driven by the disc at a constant ratio of rotation, andan amplifier for amplifying the output voltage of the potentiometer.

According to another embodiment of the invention, utilising ordinarylinear-law electrical components to provide greater exactness the higherthe value of x, the first generator includes two linear potentiometersdriven simultaneously at a constant ratio of rotation by the disccontrolling the displacement of the grid, each potentiometer beingconnected to the input of an operational amplifier, the twopotentiometer-amplifier combinations being connected in series.

According to another embodiment of the invention, to enable measurementof a diagonal of an impression of which the dimensions exceed thosewhich correspond to the normal travel of the disc controlling thedisplacement of the grid, an additional resistor is interposed betweeneach potentiometer and the associated amplifier, the terminals of eachof these two resistors being connected to a short-circuiting switch, thetwo switches being in turn coupled to a common operating device.

In another embodiment to obviate the need for manualsetting of thepotentiometer or potentiometers of the first generator to zero the firstgenerator includes a movable member coupled to the disc and displaceablein a housing, the housing being provided with means for adjusting itsposition relative to the frame of the microdurometer, the position ofthe movable member relative to the housing, corresponding with zerooutput voltage, being determined by a stop.

Further features and advantages of the present invention will becomeapparent from the following description of embodiments thereof, given byway of example only, with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a schematic representation of an apparatus for hardnessmeasurement, provided with a built-in device for optical observation ofthe impression.

FIG. 2 diagrammatically shows part of the microdurometer for automatichardness measurement,

FIG. 3 diagrammatically shows an embodiment of the two generators ofFIG. 2,

FIG. 4 diagrammatically shows another embodiment of the two generatorsof FIG. 2,

FIG. 5 is a general perspective view of the microdurometer,

FIG. 6 diagrammatically shows another embodiment of the parts of themicrodurometer shown in FIG. 2,

FIG. 7 diagrammatically shows in more detail the parts shown in FIG. 6,

FIG. 8 shows the projection of the impression in the plane of thesighting grid and the correspondingpositions of the disc, formeasurement of an impression of which the length exceeds the normaltravel of the disc, where 8a represents a grid position used forreference, 8b represents a left side grid position, measurement, andrepresents a right side grid position measurement,

FIG. 9 diagrammatically shows a modification of the parts shown in FIG.7, and

FIG. 10 diagrammatically shows means for automatic reset to zero of thefirst generator of FIGS. 7 or 9 Referring to FIG. I, the microdurometercomprises a slide 1 guided in its movement by members here representedschematically by rollers 2. The slide 1 carries at one end a pyramidaldiamond 3. The application of the diamond 3 to the sample 4 is obtainedin accordance with an automatic cycle by the rotation of a cam 6 againstwhich bears a lever 7 pivoted at a fixed point 8. The lever bears at aknife-edge 9 against a beam 10 loaded at one end with a weight 11, andbearing at its other end against a knife-edge 12 on the slide 1. It canbe seen that when the cam 6 has lifted the lever 7 sufficiently to bringthe diamond 3 into contact with the sample 4, the force of penetrationof the diamond 3 results from the weight of the known mass 11, possiblymultiplied by a coefficient resulting from the ratio of the leverage ofthe arms of the beam 10.

When the slide and the diamond have returned to the position of restshown in the figure, the impression made by the diamond on the sample 4can be observed through a conventional lens-system by means of eyepiece16 and mirror 17. The impression is illuminated by a luminous source 18via a semi-reflecting mirror 19.

The length of the diagonal of the impression is measured by measurementof its image in the object plane of the eyepiece 16 by paralleldisplacement of an engraved grid 20 by rotation of an engraved circularscale 21 of which the angular rotation is proportional to the lineardisplacement of the grid 20.

In known microdurometers the length of the diagonal is measured byoptically aligning the grid with each end of the image of theimpression, the measurement resulting from the reading of thecorresponding rotation on the circular scale. The applied load resultingfrom the mass 11 loading the end of the beam 10 is known.

Referring now to FIG. 2, in an embodiment of a microdurometer inaccordance with the invention, the circular scale 21 controlling thedisplacement of the grid 20 is coupled by a positive transmissionmechanism 22 to a rotary potentiometer 24.

The potentiometer 24 is an a=l/x law potentiometer, that is, its outputvoltage varies as the inverse square of its angular displacement.

The potentiometer 24 is fed at stabilized voltage by a generator 25, andthe output voltage at the cursor 24 of the potentiometer is amplified bythe amplifier 27.

The potentiometer 24 and a voltage generator are connected in series toprovide analogue product of their output voltages with the result thatthe output voltage from the generator30 represents the product of thevoltages produced in each of the generator and potentiometer.

This output voltage is amplified in an amplifier 31 and fed to a digitalvoltmeter 32.

It can be seen that if, when the grid is aligned with one of the ends ofthe impression on the sample 4, the potentiometer 24 is brought to zerooutput voltage by turning its housing 26 by means of a knob 34, theoutput voltage at the cursor 24 at the time of the second alignment ofthe grid with the other end of the diagonal of the impression will beproportional to l/d", d being the length of the diagonal of theimpression. The generator 30 is a linear generator, the voltage producedbeing proportional to the load 11. It can be seen that under theseconditions the voltage at the output of the amplifier 31 is bothproportional to the load and inversely proportional to the square of thelength of the diagonal of the impression. The voltage measured by thevoltmeter 32 will thus be, except for a constant coefficient, themeasure of the hardness of the sample 4, and the voltmeter can begraduated directly in a scale of hardness. In practice the voltmeter isof dual sensitivity selected by means of a changeover switch 35, andcorresponding with two hardness scales, Vickers and Knoop. The voltmeter32 is followed by a print-out recorder 36.

FIG. 3 shows one form of the measurement circuit which has beendescribed above. Here the source of stabilized voltage 25 is simplyconstituted by a battery, and the generator 30 is made of four straingauges connected together in a Wheatstone bridge. The group of gauges isglued to a rod by which a pan carrying the weight 11 hangs from the beam10.

FIG. 4 shows another form of the generator 30. Here a generator 30simply consists of a step-potentiometer fed by the output voltage fromthe amplifier 27. Each step corresponds with one of the weights in theseries used for loading the beam 10, in such a way that the voltage dropin the potentiometer is proportional to the weight corresponding withthe reference number of the contact stud.

Referring now to FIG. 5 which represents in a simplifled manner theexternal appearance of an embodiment of a microdurometer in accordancewith the invention, the known members ofa microdurometer of aconventional type will be recognised. Thus the sample 4 of which it isdesired to measure the hardness is held onto the table by a clamp 41.The eyepiece 16 is incorporated in a knurled knob 42 which controls therotation of the graduated circular scale 21 and at the same time thelinear displacement of the sighting grid. Inside the apparatus a pair ofgears 44 and 45 transmit the rotational movement of the circular scaleto the potentiometer 24. As previously stated, the knob 34 allowsrotation of the housing of the potentiometer 24 for resetting the outputvoltage and hence the reading on the voltmeter 32 to zero at the time offirst aligning the grid with one end of the diagonal of the impressionmade on the sample 4.

The apparatus is here equipped with the strain gauge type of generator30 which is inserted between the beam 10 and the pan carrying theapplied load 11. The electrical connection from the generator 30 to theother apparatus of the measurement equipment is ef' fected by the cable36. The final measurement signal is transmitted to the digital voltmeter32 through the cable 37.

When using the above described apparatus the changeover switch 35 isfirst switched to the position corresponding to the nature of thediamond with which the slide has been equipped. Then an impression ismade on the sample. The grid is next brought into alignment with one ofthe ends of the impression and the reading of the voltmeter 32 isbrought to zero by means of the knob 34. Next when the grid is broughtinto alignment with the other end of the diagonal of the impression, thevalue of the hardness of the sample is automatically displayed on thevoltmeter, without having to make any calculations.

If the apparatus is equipped with a step-potentiometer as the generator30, it is necessary before aligning the grid with the impression, to setthe potentiometer on the step corresponding to the load 11 put on thepan.

Referring to FIG. 6, there is shown another embodiment of themeasurement circuit. It will be observed that the graduated circularscale 21, mechanically coupled in other respects to the means fordisplacing the grid, is also mechanically coupled to the cursors of twolinear potentiometers l0 and 51. Each linear potentiometer and 51 isconnected respectively to the input of two operational amplifiers 52 and53. The two potentiometer-amplifier groups 50 52 and 51 53 are connectedin series.

The diagram of FIG. 7 shows in more detail reverse feedback resistors 64and 65 of each amplifier 52 and 53. The two amplifiers, their tworeverse feedback resistances and the two potentiometers haverespectively the same characteristics. If the value of the reversefeedback resistor 64 or 65 is designated by R and the value of theresistance of the potentiometer 50 or 51 by r, it can be seen that thegain g of each amplifying stage is equal to R/r. Now r is proportionalto the deviation x of the graduated circular scale, that is to say, tothe displacement d of the grid. Consequently the gain g of each stage isequal to R/Ad and the overall gain G of the two stages in series isequal to R /A d which can also be written G=I(/d since R and A areconstants. The input voltage U, provided by the source 25 being fixed,the voltage U, at the output from the amplifier 53 will be equal to U KMThus it will be sufficient to adjust the gains of each amplifier 52, 53,27 and 31 to obtain the law H=Kl'*/a allowing direct measurement of thehardness.

Referring now to FIG. 9 there is shown yet another embodiment of themeasuring circuit. It will be seen that the circuit differs from that ofFIG. 7 by the addition of fixed resistors 59 and 60 and switches 61, 62.Resistor 59 is interposed in series between the potentiometer 50 andamplifier 52, and its value is equal to the maximum resistance of thepotentiometer 50. Similarly the resistor 60 between the potentiometer 51and the amplifier 53 has a value equal to the maximum resistance of thepotentiometer 51. Each of the two resist ances 59 and 60 can beshort-circuited respectively by the switches 61 and 62, which aremechanically coupled.

In the case of the circuit of FIG. 7, or in the case of the circuit ofFIG. 9 with the switches 61 and 62 closed, the maximum measurable lengthof the impression diagonal corresponds to a rotation slightly less thanone turn of the circular scale 21, that is to say, to the maximum travelof the potentiometers 50 and 51. Let us assume that the maximum travelof the circular scale corresponds to a displacement of the grid by 100microns. We know that in this case the measurement of the diagonal ofthe impression is made by sighting the grid on one end of the diagonaland bringing a pointer opposite the zero of the graduation of thecircular scale, then making another signting of the grid on the otherend of the diagonal.

In the case where the dimension of the diagonal exceeds 100 microns, therefinement of FIG. 9 allows its measurement to be made, by employing agrid with two engraved lines 55 and 56 (FIG. 8), the distance betweenthe two lines being exactly 100 microns, and the positive sense of themeasurements corresponding with the sense of displacement of 56 towards55. The measurement operation is then performed in three phases shown bythe three parts 8a, 8b and 8c of FIG. 8. Considering these three phasesin terms of the manual operation of the apparatus, in the first phasethe righthand line 55 of the grid is centred approximately in the middleof the impression, and the pointer is adjusted to zero on the circularscale (FIG. 8a). In the second phase (FIG. 8b) the lefthand line 56 ofthe grid is sighted on the lefthand end of the impression and thereading :1, is taken from the circular scale. This reading is more than50 microns since the impression is assumed to measure more than 100microns. In the third phase (FIG. 8c) the righthand line 55 of the gridis finally sighted on the righthand end of the diagonal, and thecorresponding reading d is observed, which is also larger than 50microns. The dimension d of the diagonal of the impression expressed inmicrons will then be a'= l00+d -d,.

Thus it can be seen that in relation to direct measurement of theimpression, a constant of 100 microns has been added to d, correspondingwith the distance between the two grid lines 55, 56. It is the constantresistances 59 and 60 which correspond to the increase by the 100 micronconstant, since each of these resistances is equal to the maximum valueof resistance of the potentiometers 50 and 51, and these potentiometersallow coverage of a range of 100 microns.

It may be observed that the function thus effected by the circuit fromthe input to the potentiometer 50 up to the output from the amplifier 53is: output voltage U, U K/(l0IH-a' FIG. 10 shows an arrangement of partof the circuit of FIGS. 7 or 9 whereby manual resetting of thepotentiometers 50, 51 to zero is avoided. The two potentiometers 50 51are enclosed in a common housing 26 provided with a flange 70. Thehousing is mounted in the frame of the microdurometer in an entirelyconventional manner so that it is free to rotate relative to the frame.For this reason, and in order to simplify the drawing, the free mountingof the housing in the frame is not shown. The housing 26 can be fixedrelative to the frame or released by adjustment of stop-screw 71threaded into a part 72 of the frame with its end 73 able to bearagainst the flange to fix it against rotation.

Otherwise the shaft 22, integral with the movable parts of thepotentiometers, is coupled to the flange 70 by a spiral spring 75 fixedat 76 to the shaft 22 and at 77 to the flange. The shaft 22 also has across spur 78 while the flange 70 bears a stop 79. The action of thespiral spring tends to bring the spur and the stop into contact, and thewhole has been adjusted so that this stopped position corresponds withthe electrical zero position of the potentiometers.

With such an arrangement the operation of hardness measurement can beperformed in a simpler and more exact manner. The flange 70 being fixedby the screw 71, one proceeds in a normal manner to sight the grid onone of the ends of the impression. This entails a rotation of thecircular scale 21, of the internal movable parts of the potentiometersand the spur 78. Without interrupting the sighting it is then sufficientto release the screw 71, which frees the flange 70 and the potentiometerhousing; the spring 75 then automatically brings the stop 79 on thehousing into contact with the spur 78, thus setting the resistance ofthe potentiometers to zero. The operator, still without having left thesighting, clamps the flange 70 and the housing in this zero position,then proceeds to sight the grid on the other end of the impression, thedisplay on the voltmeter then corresponding with the hardnessmeasurement.

Of course the invention is not intended to be limited to the embodimentwhich has been described but covers all the variants which originatefrom it.

It may be observed, for example, that the device for resetting thepotentiometers 50, 51 to zero after the first sighting, described withreference to the apparatus with the two Iinerar potentiometers, would beapplicable in the same manner to the apparatus with a nonlinearpotentiometer. The recall spring 75 could be omitted, the movement ofthe housing towards the stop position being effected manually; theresult is thus attained with almost as much advantage, the stop allowingthe housing and the movable member to be brought into the zero startingposition without having to perform a visual check at the voltmeter.Likewise the circuit could be energised with alternating current whichwould allow exemption from continuous drift in the d.c. amplifiers. Thesource 25 would then be a pilot oscillator of constant frequency andamplitude; the remainder of the electronic circuit would be practicallyunchanged, but the voltmeter would obviously be an alternatingvoltmeter.

What we claim is:

l. A microdurometer for measuring the hardness of a sample by theproduction of an impression therein by means of a pyramidal impressdiamond with four faces applied to the sample under a force P, in whichthe measurement of the hardness H is determined by a formula of the formH=KP/d d being the length of the diagonal of the impression and K aconstant coefficient, the microdurometer comprising a slide carrying animpress diamond, operating means for the slide for applying the diamondto a sample with an adjustable force, optical means for observing theimpression, means for linearly displacing a sighting grid observablethrough said optical means, a first voltage generator coupled to thedisplacing means, means independent of said displacing means for settingthe output signal of said first generator to zero for use as a referencewhen said sighting grid is at a first location, whereby the firstgenerator provides an output signal in response to displacement of thesighting grid to a second location, a second voltage generator forproviding an output signal proportional to the force of application ofthe diamond, the two' generators being connected to provide an analogueproduct of their output signals, and a voltmeter for indicatingtheproduct of the two signals.

2. A microdurometer as in claim 1, in which the displacement of the gridis controlled by a rotatable disc whose angular rotation is proportionalto the linear displacement of the grid, and the first generator includesan a=l/x "law potentiometer, driven by the disc at a constant ratio ofrotation, and an amplifier for amplifying the output voltage of thepotentiometer.

3. A microdurometer as in claim 1, wherein the second generatorcomprises strain gauge means connected in a Wheatstone bridge which isconnected to be energised by the output signal from the first generatorand mounted on a member transmitting the applied force, and an amplifierfor amplifying the output voltage from the Wheatstone bridge.

4. A microdurometer as in claim 1, in which the operating means includea pan which can be loaded with a series of stepped weights for varyingthe force of application of the diamond and the second generatorincludes a step-potentiometer, each step corresponding to one of thestepped weights, the stepping of the resistances in the potentiometerbeing inversely proportional to the stepping of the weights.

5. A microdurometer as in claim 1 wherein the first generator is anon-linear potentiometer and the means for reducing the output signalfrom the first generator to zero includes a device for rotationaladjustment of the housing of the non-linear potentiometer.

6. A microdurometer as in claim 1, in which the displacement of theimpression sighting grid is controlled by a rotatable disc whose angularrotation is proportional to the linear displacement of the grid and thefirst generator is adapted to provide an output signal inverselyproportional to the square of the linear displacement of the grid andincludes two linear potentiometers driven simultaneously at a constantratio of rotation by the disc, each potentiometer being connected to theinput of an operational amplifier, the two potentiometer-amplifiercombinations being connected in series.

7. A microdurometer as in claim 6, in which an additional fixed resistoris interposed between each potentiometer and the associated amplifier,each of the two resistors being connected across a short-circuitingswitch, the two switches being coupled to a common operating means.

8. A microdurometer as in claim 7, wherein each additional fixedresistor has a value equal to the maximum value of the resistance of thecorresponding potentiometer.

9. A microduromater as in claim 1, in which the displacement of the gridis controlled by a rotatable disc whose angular rotation is proportionalto the linear displacement of the grid and the first voltage generatorcomprises a movable member coupled to the disc and displaceable in ahousing, the housing being provided with means for adjusting itsposition relative to the frame of the microdurometer, the originposition of the movable member relative to the housing being determinedby a stop.

10. A microdurometer as in claim 9, wherein the means for adjusting thehousing relative to the frame comprises a releasable locking device.

11. A microdurometer as in claim 10, wherein the movable element and thehousing are coupled by an elastic recall device tending to bring them totheir relative origin position.

1. A microdurometer for measuring the hardness of a sample by theproduction of an impression therein by means of a pyramidal impressdiamond with four faces applied to the sample under a force P, in whichthe measurement of the hardness H is determined by a formula of the formH KP/d2, d being the length of the diagonal of the impression and K aconstant coefficient, the microdurometer comprising a slide carrying animpress diamond, operating means for the slide for applying the diamondto a sample with an adjustable force, optical means for observing theimpression, means for linearly displacing a sighting grid observablethrough said optical means, a first voltage generator coupled to thedisplacing means, means independent of said displacing means for settingthe output signal of said first generator to zero for use as a referencewhen said sighting grid is at a first location, whereby the firstgenerator provides an outpuT signal in response to displacement of thesighting grid to a second location, a second voltage generator forproviding an output signal proportional to the force of application ofthe diamond, the two generators being connected to provide an analogueproduct of their output signals, and a voltmeter for indicating theproduct of the two signals.
 2. A microdurometer as in claim 1, in whichthe displacement of the grid is controlled by a rotatable disc whoseangular rotation is proportional to the linear displacement of the grid,and the first generator includes an ''''a 1/x2''''-law potentiometer,driven by the disc at a constant ratio of rotation, and an amplifier foramplifying the output voltage of the potentiometer.
 3. A microdurometeras in claim 1, wherein the second generator comprises strain gauge meansconnected in a Wheatstone bridge which is connected to be energised bythe output signal from the first generator and mounted on a membertransmitting the applied force, and an amplifier for amplifying theoutput voltage from the Wheatstone bridge.
 4. A microdurometer as inclaim 1, in which the operating means include a pan which can be loadedwith a series of stepped weights for varying the force of application ofthe diamond and the second generator includes a step-potentiometer, eachstep corresponding to one of the stepped weights, the stepping of theresistances in the potentiometer being inversely proportional to thestepping of the weights.
 5. A microdurometer as in claim 1 wherein thefirst generator is a non-linear potentiometer and the means for reducingthe output signal from the first generator to zero includes a device forrotational adjustment of the housing of the non-linear potentiometer. 6.A microdurometer as in claim 1, in which the displacement of theimpression sighting grid is controlled by a rotatable disc whose angularrotation is proportional to the linear displacement of the grid and thefirst generator is adapted to provide an output signal inverselyproportional to the square of the linear displacement of the grid andincludes two linear potentiometers driven simultaneously at a constantratio of rotation by the disc, each potentiometer being connected to theinput of an operational amplifier, the two potentiometer-amplifiercombinations being connected in series.
 7. A microdurometer as in claim6, in which an additional fixed resistor is interposed between eachpotentiometer and the associated amplifier, each of the two resistorsbeing connected across a short-circuiting switch, the two switches beingcoupled to a common operating means.
 8. A microdurometer as in claim 7,wherein each additional fixed resistor has a value equal to the maximumvalue of the resistance of the corresponding potentiometer.
 9. Amicroduromater as in claim 1, in which the displacement of the grid iscontrolled by a rotatable disc whose angular rotation is proportional tothe linear displacement of the grid and the first voltage generatorcomprises a movable member coupled to the disc and displaceable in ahousing, the housing being provided with means for adjusting itsposition relative to the frame of the microdurometer, the originposition of the movable member relative to the housing being determinedby a stop.
 10. A microdurometer as in claim 9, wherein the means foradjusting the housing relative to the frame comprises a releasablelocking device.
 11. A microdurometer as in claim 10, wherein the movableelement and the housing are coupled by an elastic recall device tendingto bring them to their relative origin position.